One reason for your Processor Serial Number to be invalid is if you purchased the OEM/TRAY version of the processor.
Your processor should have come in a Official Sealed AMD Retail Box sealed with a white sticker with the processor model number and Retail Serial Number.
If it came in a different Non-AMD Retail box or just in a clamshell and nothing else then it is most likely the OEM/TRAY version.
If your Processor did come in a Sealed AMD Retail Box then you need to see Photos of the Processor's top where it shows the Model and Serial Numbers on it.
If it is a fake, someone could have changed the CPU lid and replaced it from another processor.
I would open a AMD Service Request -Warranty and find out why the Serial Number was rejected from here: https://www.amd.com/en/support/contact-email-form
Can you post an Image of the top and bottom (showing the pins) of your processor?